1. Field of the Invention
This invention relates to a fermentation process useful in the production of ethanol from renewable plant biomass. An ever-increasing interest in the development of such processes has stemmed from the current emphasis on utilizing ethanol as an alternative liquid fuel.
Plant solids contain three major components: lignin, cellulose, and hemicellulose. The lignin is a polyphenolic macromolecule which binds together the cellulose and adds rigidity to the plant material. It is not convertible to alcohol. The cellulose is a homopolymer of glucose, and upon separation from the lignin yields a hydrolyzable source of fermentable hexose sugar. Hemicellulose comprises up to about 35% of the plant solids and is readily hydrolyzed with dilute acid under mild conditions to a mixture of sugars, with D-xylose (a pentose) as the major product. However, the lack of an effective fermentation process to use D-xylose has seriously diminished the practicality of plant biomass as a source of petroleum-sparing compounds.
2. Description of the Prior Art
Bacteria and fungi have previously been recognized for their ability to convert pentoses to ethanol or at least have a participating role in the conversion pathway. A review S. L. Rosenberg ["Fermentation of Pentose Sugars to Ethanol and Other Neutral Products by Microorganisms," Enzyme Microbiol. Technol. 2: 185-193 (1980)] teaches, for example, that Fusarium oxysporium in resting cell suspension will ferment D-xylose to approximately equimolar amount of ethanol, CO.sub.2, and acetic acid. In growing cultures, the proportion of ethanol and CO.sub.2 to acetate is somewhat increased. Certain bacterial species of Escherichia, Spirochaeta, Aeromonas, bacillus, and Aerobacter are also shown to produce ethanol from pentose sugars, but in relatively low yield and usually in conjunction with 2,3-butanediol and a variety of organic acids. G. C. Avgerinos et al. ["A Novel Single Step Microbial Conversion of Cellulosic Biomass to Ethanol," presented at the International Frementation Symposium, London, Ontario, Canada, July 1980] reports an attempt to convert corn stover directly to ethanol by means of a mixed culture of mutant strains of Clostridium thermocellum and C. thermosaccharolyticum. While the latter strain proved effective in utilizing the pentosans produced by the C. thermocellum, the overall alcohol yield was only 50% of the theoretical maximum.
It has long been recognized that yeasts have the ability to stoichiometrically convert 1 mole of glucose to 2 moles each of ethanol and CO.sub.2. Also, yeasts are able to assimilate natural pentoses oxidatively, but they are not able to ferment them to ethanol as they can glucose (Rosenberg, supra, paragraph bridging pages 187 and 188). Several yeasts have recently been shown to have the capacity for fermenting D-xylulose, a keptopentose. In connection therewith, Wang et al. [Biotechnol. Lett. 2(6): 279-284 (1980)] demonstrated the feasibility of adding glucose isomerase to a D-xylose-containing medium, thereby converting the substrate in situ to the fermentable xylulose form. However, the ultimate rate of ethanol production was low and the yield was a mere 10% of the theoretical. Gong et al. [Appl. Environ. Microbiol. 41(2): 430-436 [February 1981)] reportedly bolstered the yield by this technique to greater than 80%, but on a commercial scale the enzyme-assisted conversion will most likely remain economically unattractive.